CD8+ T cells play a major role in controlling HIV-1 infection, but the sole presence of these cells is not sufficient to prevent disease progression. Certain qualities or characteristics among these different HIV-1 specific CDS+ T cell responses must therefore explain the differences in disease progression seen in infected individuals. The overall goal of this proposal is to understand the mechanisms leading to the eventual failure of CD8+ T cells to control infection. The maturation phenotype of HIV-1 specific CD8+ T cells, as delineated with surface marker staining, varies significantly among infected subjects on antiretroviral therapy (ART). As expected, HIV-1 specific CD8+ T cells from some individual on ART progressed to a memory phenotype; however, others did not despite that fact that all individuals had attained undetectable plasma VL. Our first specific aim hypothesizes that HIV-1 specific CD4+ T cells are necessary for the maturation of CDS+ T cells. We will measure HIV-1 specific CD4+ T cell responses using intracellular cytokine staining and compare this to the maturation phenotype of HIV-1 specific CD8 cells based on staining of tetramer positive cells with antibodies to CD45RA, CD27, and CD28 in a cohort of subjects before and after the implementation of ART. Our second specific aim is grounded on the fact that certain HLA class I alleles can be used to predict the course of HIV-1 infection. We found that patients carrying the HLA-B+57 allele have a lower plasma viral load when compared to the rest of the cohort. We went on to show that these individuals have CD8 + T cell responses that target highly conserved regions in Gag. We therefore hypothesize that HLA-B+57 restricted epitopes in HIV-1 infected subjects have few cytotoxic CD8+ T lymphocyte escape mutations when compared with epitopes restricted by HLA-B+35 or B'53 (associated with a more rapid disease progression). We will sequence virus from infected patients who carry the alleles of interest to look for potential CTL escape mutations. Peptides will then be synthesized based upon these mutations and used to stimulate PBMC from which the epitope sequence was derived to test for the presence of CTL escape. CTL escape mutations will then be compared with markers of disease progression longitudinally and cross-sectionally among the cohort. Our third specific aim will determine whether viral functional constraints explain the differences seen in disease progression among HLA-B+35, B'53, and B'57 carriers. We will amplify the gag region from subjects found to have CTL escape mutations at epitopes restricted by these HLA class I alleles and shuttle this part of the genome into an infectious molecular clone. Infectious virus derived from this clone will subsequently be tested for its replication kinetics in a PHA stimulated PBMC assay. Our study is therefore designed to expand upon novel preliminary studies in order to gain a better understanding of the determinants of HIV-1 disease progression. Fully defining the determinants of a memory phenotype in CD8+ T cells (specific aim 1) would have important implications for ART interruption and HIV-1 vaccination (both therapeutic and preventative). An HIV-1 vaccine would also be enhanced by identifying CD8+ T cell epitopes that portend a favorable prognosis and are difficult for the virus to mutate (specific aims 2 and 3).